How to fix the engine code PO306 on 2002 Chevrolet Express 1500

First problem code PO300, repairs made, replaced plugs, wires, rotor and cap. Problem went away for two weeks and code PO300 re-appeared. Repaired made replaced the CMP sensor. The engine was fine for a week or so, the check engine light returned, found code PO306, and PO507, found and replaced defective vacuum hose. Code PO507 cleared but PO306 failed. Engine runs fine, comp. is okay, no noise from engine, smooth acce. starts fine. The problem is the misfire in the # 6 cylinder.

It is possible the engine in your 2002 Chevrolet Express 1500 has a worn valve seat on the #6 cylinder. This can be a difficult problem to correctly diagnose. It is sometimes helpful to perform a cylinder leak down test but that may check out OK also. If no testing will confirm this fault, it may be necessary to remove the cylinder head for inspection. This issue is listed in RepairPal's problem reports for your vehicle, here is the link - http://repairpal.com/worn-valve-seats-may-cause-engine-misfire-152.